The present invention relates to a method for manufacturing a disintegrative core for high pressure casting, such as die casting or squeeze casting. More particularly, the present invention relates to the manufacture of complex disintegrative cores from water-soluble salts. Also, the present invention relates to such water-soluble salt cores. In addition, the present invention is concerned with a method for extracting such water-soluble salt cores.
Generally, a core preparation technique is necessary to prepare cast articles which have complex internal structures or are undercut.
In the case of gravity casting, a disintegrative core made of hard sand or ceramic powder or a water-soluble salt core is positioned inside a mold, and then a molten metal is introduced and solidified in the mold. After that, the disintegrative core is removed by mechanical and chemical methods or the salt core is melted-out with water or steam.
In the case of a piston for an internal combustion engine, the salt core preparation technique is useful in forming a circular oil cooling gallery inside of the piston.
U.S. Pat No. 3,645,491, the content thereof being incorporated herein by reference, discloses a core preparation technique in which a powdered water-soluble salt is combined with about 10% of synthetic resin as a binding agent U.S. Pat. No. 4,629,708, the content thereof being incorporated herein by reference, discloses a core preparation technique in which a water-soluble salt, such as sodium chloride and potassium chloride, is mixed with ceramics, such as alumina, and silicone resin as a binding agent and sintered.
Under actual circumstances, however, a piston is subjected to high pressure casting, such as die casting and squeeze casting, in order to prepare a high performance aluminum alloy or composite material piston.
In such a high press casting, a conventional disintegrative core made of sand or a conventional salt core cannot be applied to a high pressure casing method because the molten metal penetrates into the inside of the core by cast pressure or the core is collapsed by high pressure.
Recently, there are developed several kinds of core preparation techniques capable of being used where high pressure casting is required. For instance, U.S. Pat. No. 3,963,818, the content thereof being incorporated herein by reference, discloses a core preparation technique in which a powdered water-soluble salt, such as sodium chloride and potassium chloride, is added with about 1% of water, molded under a high pressure of 1.8xcx9c4.0 ton/cm2, then sintered at 100xcx9c300xc2x0 C. for 20 minutes.
U.S. Pat. No. 4,438,804, the content thereof being incorporated herein by reference, discloses a core molding method in which a water-soluble salt powder is mixed with hard powder such as Zircon sand, and molded, along with potassium carbonate, barium carbonate or alkali silicate as a binding agent, under a high pressure.
U.S. Pat. No. 3,407,864, the content thereof being incorporated herein by reference, discloses a core molding method in which a water-soluble salt powder, such as sodium chloride and potassium chloride, is mixed with 3 wt % of borax, 1 wt % of magnesium oxide and 1 wt % of talc, and molded under a high pressure.
G.B. Pat. No. 2,156,720, the content thereof being incorporated herein by reference, discloses a core preparation technique in which a powdered water-soluble salt is hydrostatic-pressurized.
In these preparation methods using high pressures or hydrostatic pressures, voids between particles are minimized and their bonding force is strengthened so as for the core to keep its shape unchanged at a cast pressure of 5,000xcx9c20,000 psi and so as to prevent the molten metal from penetrating into the inside of the core.
However, these conventional methods suffer from disadvantages in that their application is restricted by the size and shape of cores and the production cost is increased because particle sizes of salt powders are required to be finely controlled.
In addition, it takes a significant amount of time to completely remove cores from the high pressure cast articles because the cores are dissolved with water.
Meanwhile, U.S. Pat. Nos. 4,446,906, 4,875,517, and 5,303,761, the contents thereof being incorporated herein by reference, disclose core preparation methods in which a water-soluble salt such as sodium chloride and potassium chloride are heated, melted, and subjected to die casting, optionally in combination with hard particles, such as silica and alumina.
Somewhat superior as it is to molding methods in core shape and productivity, these conventional methods are limited in their application by the size of the core. In addition, the conventional methods require a significant amount of time to completely remove cores from high pressure cast articles when water is used to dissolve the cores.
U.S. Pat No. 4,840,219, the content thereof being incorporated herein by reference, discloses a method in which a molten salt of a mixture comprising 40% by weight of NaCl and 40% by weight of Na2CO3 is added with 10xcx9c50% of hard powder to give a slurry which is introduced into a mold. U.S. Pat. No. 3,459,253, the content thereof being incorporated herein by reference, discloses a method in which wire or glass fiber is added to the molten mixture salt comprising sulfate salt and carbonate salt to give a slurry which is introduced into a mold.
These methods have more variety in shape and size of the core than does the pressurization method or the die cast method. Meanwhile, because the used salts are as high as above 660xc2x0 C. in melting temperatures, cracks are easily caused owing to the shrinkage upon solidification so that the cores become brittle and are difficult to handle. Additionally, a substantial period of time is required to remove the cores of high pressure cast articles because the cores must be dissolved by use of water, and the core salts thus obtained cannot be re-used.
In view of the aforementioned problems and considerations, it is an object of the present invention to provide a method for manufacturing a disintegrative core for high pressure casting, capable of simply manufacturing a core of a complex shape and obtaining a core for high pressure casting by use of a re-usable aluminum alloy or a magnesium alloy.
It is another object of the present invention to provide a disintegrative core for high pressure casting.
It is a further object of the present invention to provide a method for extracting the core.
In one embodiment, one of the objects of the present invention is realized by the method for manufacturing the disintegrative core for high pressure case wherein a water-soluble salt, alone or in combination with a fine hard powder, is melted and solidified in a core mold; or processed into a fine powder and molded in a core mold under pressure, said water soluble salt ranging from 280 to 520xc2x0 C. in melting point and from 9.8xc3x9710xe2x88x922 to 1.2xc3x9710 W/mxc2x7xc2x0 C. in heat transfer coefficient (xcexa) with a high latent heat, whereby the disintegrative core is manufactured from the water-soluble salt.
In another embodiment, another object of the present invention is embodied by the disintegrative core for high pressure casting manufactured through the said method.
In a further embodiment, the other object is realized by the method for extracting the disintegrative core for high pressure casting from a high pressure cast article, wherein the core is heated to a melting temperature at which the high pressure cast article is not thermally deformed, the core melt is extracted, and the cast article is washed with water.
Hereinafter, the present invention describes a disintegrative core for high-pressure casting, and methods for manufacturing and extracting the same.
A disintegrative core for high pressure casting is manufactured from a water-soluble salt, wherein the water-soluble salt, alone or in combination with a fine hard powder, is melted and solidified in a core mold; or processed into a fine powder and molded in a core mold under a pressure, said water soluble salt ranging from 280 to 520xc2x0 C. in melting point and from 9.8xc3x9710xe2x88x922 to 1.2xc3x9710 W/mxc2x7xc2x0 C. in heat transfer coefficient (xcexa) with a high latent heat, whereby the disintegrative core can be applied where a light metal such as aluminum alloy or magnesium alloy is subjected to high pressure casting, such as die casting or squeeze casting.
As having a melting point of 280 to 520xc2x0 C. and a heat transfer coefficient (xcexa) of 9.8xc3x9710xe2x88x922 to 1.2xc3x9710 W/mxc2x7xc2x0 C. and being high in latent heat, the water-soluble salt is selected from the group consisting of potassium nitrate (KNO3), potassium nitrite (KNO2), sodium, nitrate (NaNO3), sodium nitrite (NaNO2), copper chloride (CuCl2), sodium chloride (NaCl), potassium chloride (KCl), lithium chloride (LiCl), lead chloride PbCl2), magnesium chloride (MgCl2), barium chloride (BaCl2), calcium chloride (CaCl2) and mixtures thereof.
In regard to melting point, 333xc2x0 C. is known for potassium nitrate (KNO3), 290xc2x0 C. for potassium nitrite (KNO2), 308xc2x0 C. for sodium nitrate (NaNO3) and 270xc2x0 C. for sodium nitrite (NaNO2).
315xc2x0 C. is measured to be the melting temperature for the mixture salt comprising, by weight ratio, 82:17 NaCl:CuCl2, 320xc2x0 C. for the mixture salt comprising, by weight ratio, 92:8 KNO3:KCl, 320xc2x0 C. for the mixture salt comprising, by weight ratio, 54:46 KCl:LiCl, 410xc2x0 C. for the mixture salt comprising, by weight ratio, 93:7 PbCl2:NaCl, 430xc2x0 C. for the mixture salt comprising, by weight ratio, 54:44 MgCl2:NaCl, 450xc2x0 C. for the mixture salt comprising, by weight ratio, 53:47 CaCl2:BaCl2 and 510xc2x0 C. for the mixture salt comprising, by weight ratio, 54:46 NaCl:CaCl2, and each salt ranges from 9.8xc3x9710xe2x88x922 to 1.2xc3x9710 W/mxc2x7xc2x0 C. in heat transfer coefficient (xcexa).
Such a water-soluble salt is solidified in a mold. At this time, the molten water-soluble salt is introduced into the mold at a temperature higher by about 30xcx9c80xc2x0 C. than that of its melting temperature, thereby minimizing the occurrence of cracks attributed to the shrinkage upon solidification.
In this regard, if the temperature of the molten water-soluble salt is above 80xc2x0 C., shrinkage-attributable cracks and pores are generated upon solidification. On the other hand, if the temperature is below 30xc2x0 C., the water-soluble salt is difficult to inject into a mold.
In addition, the mold temperature is controlled not to exceed half of the melting point of each salt to be introduced. The reason is that, if the mold temperature is lower, the salt is not well introduced into the mold. On the other hand, if the temperature of the mold is higher, the surface structure of the solidified core becomes so coarse that a thermal deformation occurs in deep parts of the core upon high pressure casting.
Accordingly, suitable for mold material is graphite, which is excellent in thermal conductivity. When the mold is made of graphite, the molten salt is easily introduced into the mold and the solidification rate becomes so fast that the surface texture of the solidified core is made fine.
In the molten salt, fine thermostable hard particles such as powders, fibers and whiskers of chemically non-reactive metals or ceramics, may be added. For use, they are homogeneously dispersed.
By way of examples, and not limitation, suitable for metal particles is silicon with a high solidity and a similar specific gravity to that of the salt Also, alumina (Al2O3), silicon carbide (SiC) and so on can be used as the ceramic particles. Such fibers or whiskers can be used.
The hard particles are preferably added at an amount of 5xcx9c30 wt %. If the added amount of the hard particles is above 30 wt %, the shrinkage upon solidification can be inhibited and a strength of the core attributed to dispersion effect becomes high. But it suffers from a problem of the hard particles partly adhering onto a surface of a high pressure cast article. On the other hand, if the added amount is below 5 wt %, the addition effect of the hard particles cannot be obtained.
The water-soluble salt is processed into a powder, introduced into a core mold and molded to a core. In this regard, it is extremely preferred that the water-soluble salt powder has a size of 40xcx9c100 xcexcm for the molding under pressure. Also, a lubricant not chemically reacting with the salt is preferably used so as to facilitate the separation of the cast article from the mold.
At this time, it is preferred that the molding pressure resulting from the pressurization ranges from 60 to 100 Mpa. In addition, the molded core is preferably kept at its melting point for 0.5xcx9c1 minute, so as to make the surface texture of the core fine.
The core obtained according to the method of the present invention can be used where a metal alloy such as an aluminum alloy or a magnesium alloy is subjected to high pressure casting.
Upon the high pressure casting of a metal with a low thermal capacity, such as an aluminum alloy and a magnesium alloy, the mold is instantly filled with the melt within 0.5xcx9c3 seconds. Because the metal alloy such as an aluminum alloy or a magnesium alloy, although having a melting point (280xcx9c520xc2x0 C.) lower than a usual casting temperature (640xcx9c720xc2x0 C.) of common molten metals, has a heat transfer coefficient (xcexa) (9.8xc3x9710xe2x88x922xcx9c1.2xc3x9710 W/mxc2x7xc2x0 C.) which is only 1/1500xcx9c1/3000 of that (331xcx9c403 W/mxc2x7xc2x0 C.) of steel, which is a typical material for high pressure casting molds. The molten alloy is drastically cooled as soon as it is introduced into the mold.
In detail, because the core is lower in thermal conductivity than is steel, a material for the mold, most of the heat that the melt retains is transferred toward the mold while the beat is hardly transferred toward the core. Meanwhile, the high latent heat of the core permits a thermal deformation to occur only in the surface down to a depth of 2-3 xcexcm but not in the total shape. In other words, the core is not deformed, nor changed in its total shape by virtue of its high latent heat.
In such general casting as high pressure casting which is instantly completed, the water-soluble salts of the present invention are suitable as cores for casting metals with low thermal capacity such as aluminum and magnesium.
Meanwhile, a method for removing the core from a high pressure cast article, that is to say, a method for extracting the core from the cast article, is conducted by heating the core to a melting temperature at which the high pressure cast article is not thermally deformed, extracting the core melt, and washing the cast article with water.
When the high pressure cast article is heated at 320xcx9c550xc2x0 C. for 3xcx9c5 minutes, the heat is transferred to the inside of the core so that the core is melted and extracted. The material thus obtained can be re-used in molding of the core to bring about an economical favor.